1. Field of the Invention
The invention relates to a fill level switch for measuring the fill level of a medium in a container, having a sensor element for contacting the medium, a sender for applying an electromagnetic emission signal to the sensor element and having an evaluation unit for detecting a received signal from the sensor element, wherein the sensor element includes at least one cable for guiding the emission signal. Moreover, the invention relates to a sensor element for such a fill level switch.
2. Description of Related Art
Fill level switches have been known in industrial process measurement engineering for multiple decades and serve to detect the fill level of a medium in a container, wherein the medium affects the sensor element once it has reached a certain fill level or a certain fill level area. Due to the influence on the sensor element by the medium, a certain measured variable of the sensor element changes—depending on the embodiment of the fill level switch—and when a certain level of the reading is exceeded; a certain fill level is then considered to have been measured.
When a fill level switch is mentioned in the following, then it is not intended to be limited to the fill level switch actually triggering switching, rather it is intended, in general, that a certain fill level can be detected by comparing a reading with a—pre-settable—level and/or the fill level switch is only appropriate simply due to its construction to detect an essentially selective fill level.
The measuring principles used with fill level switches are partially based on the evaluation of mechanical or electric oscillations or simply on the evaluation of the change of a conductance value (conductive fill level switch). All of the fill level switches considered have in common that the medium of which the level is to be measured comes into contact or, respectively, has to come in contact with the sensor element.
Fill level switches that are based on the evaluation of a mechanical oscillation are desirably used, since the measuring principle is suitable for different media and is quite sturdy in respect to variations of particular media characteristics, such as, e.g., the density and viscosity of a medium. In these fill level switches, the oscillation of an oscillation fork is evaluated, which has been excited to a resonance oscillation by actuators and which is damped and also changed in its frequency by a medium surrounding the oscillation fork.
Disadvantages of fill level switches of this type, however, are that the sensor element has to be extended into the medium to be detected, the dependency of the measurement on the mounting position of the fill level switch as well as a possibly problematic cleaning of the sensor element designed as oscillation fork, since the risk of “formation of shades” is always present when spraying the inside of the container, i.e., there is a risk that the sensor element hides parts of itself and the cleaning fluid used is not able to reach all parts of the sensor element.
Other limit switches, such as, e.g., conductive limit switches extend comparably far into the medium to be detected, even when the sensor element designed as one electrode or multiple electrodes is only formed with a stub shape. The cable contained in the sensor element contacts the electrodes, wherein the emission signal usually consists of a simple alternating voltage, which—depending on the conductivity of the medium to be detected—causes measurement currents of different sizes. In such cases, in which the medium cannot be supplied with a continuously-acting alternating electric field, the use of such a conductive limit switch is prohibited; furthermore, there are also problems, here—as explained above in the oscillation fork fill level switches—in view of cleaning, which is relevant, in particular, in food and hygiene applications.